A printer, or like image forming apparatus is known in which data such as addresses output from a data storage provided in a personal computer is formed on a photosensitive member as a toner image, and the formed toner image is transferred to a sheet material such as an envelop.
In such an image forming apparatus, a cassette containing stacks of small sized sheets side by side therein is attached to the apparatus, these small sized sheets are dispensed side by side from the cassette by a feed roller. Also, a sheet sensor is arranged above each stack of sheets defined in the cassette containing stacks of sheets arranged side by side so as to detect the presence or absence of the sheets contained therein. These sheet sensors consist essentially of a photointerrupter including a light emitting element and a photodetector, and a lever which is provided rotatably between the two elements and comes to contact with the uppermost one of the corresponding stack of sheets in the cassette due to the weight thereof. These sensors are designed to determine the presence of sheet in the cassette by detecting the height of the uppermost one of sheets (the level of the sheet stack) contained in the cassette.
In the conventional apparatus of this type, when the small sized sheets are fed side by side from the cassette, the sheets are kept being fed even if one stack of sheets have run out (there is no sheet fed along one of transport paths) until the other stack of sheets runs out. Thus, a toner image to be transferred to the sheet from the stack of sheets that have run out deposits on the transfer device and the transport path downstream therefrom, thereby smearing the same. Further, the smear on the transfer device and the like deposits on a sheet fed next, thereby smearing that sheet. As a countermeasure for this, it can be considered that the sheet feeding operation is stopped if any of the sheet sensors detects the absence of sheet.
Further, the image forming apparatus can be selectively attached to a cassette containing a single stack of large sized sheets therein. These large sized sheets are dispensed one by one from the cassette by another feed roller.
There are cases where envelopes are used as sheets. Since the envelopes have end portions thereof turned up in their finished forms, the thickness thereof is nonuniform. Thus, if a cassette containing a stack of large sized envelopes therein is attached to the image forming apparatus, the nonuniformity of the thickness of that stack in a widthwise direction of the cassette becomes conspicuous. As a result, even if the sheet sensor positioned above a portion of the stack of envelopes where the thickness is large detects the presence of the sheet properly, the lever of the sheet sensor positioned above a portion of the stack where the thickness is small may rotate down as much as the sheet sensor detects the absence of sheet.
Accordingly, if the control for a cassette containing stacks of small sized sheets is employed for a cassette containing a single stack of large sized sheets, the sheet feeding operation is liable to be stopped despite the presence of sheet in the cassette. This is because there is a height difference between opposite two sides of the stack of envelopes as described above, and the control for small sized sheet stops the sheet feeding operation when any one of the sheet sensors detects the absence of sheet.